1. Field of the Invention
The invention relates to a process for producing carboxylic acid functionalized polyolefins from a grafted maleic anhydride precursor. The process involves forming a dispersion of the molten graft precursor in a liquid polar medium containing water and hydrolyzing the anhydride functionality to the corresponding carboxylic acid functionality. The invention also relates to acid functionalized polyolefin powders produced by the process and adhesive blends formulated therewith.
2. Description of the Prior Art
Polyolefins having polar functionality grafted onto the polymer backbone are widely utilized in adhesive blends used to bond layers of dissimilar materials. The grafted component is typically combined with one or more non-grafted resins to produce the adhesive blend. The amount and type of grafted and non-grafted components will vary depending on the end-use application; however, the graft component is often a graft of either an unsaturated carboxylic acid anhydride, such as maleic anhydride, or an unsaturated carboxylic acid, such as fumaric acid. The graft component typically comprises about 5 to 25 weight percent (wt. %) of the adhesive blend.
One important application for adhesive blends of the aforementioned resin types is coextrusion of multi-layer food packaging composites where the adhesives function as “tie layers” to bond the various resin layers of the composite. These resin layers include a wide variety of polar and non-polar polymers employed to provide essential properties such as structural integrity and barriers to permeation of moisture or oxygen. Commonly used resins of this type include, for example, nylon, linear low density polyethylene (LLDPE), ethylene-vinyl alcohol copolymer (EVOH) and the like.
While both maleic anhydride (MAH) and fumaric acid (trans-2-butenedioic acid) can be grafted, MAH has several advantages as a graft monomer. These include higher reactivity, lower melting point which enables it to easily be injected into a reactive extruder as a liquid, and higher volatility which allows unreacted material to be easily removed under vacuum. Also, MAH is less likely to homopolymerize in the grafting process.
As a result of the higher reactivity of MAH, significantly higher graft efficiency is obtained with MAH and it is possible to produce grafted polyolefins having higher levels of functionality at lower cost. For example, while it is possible to produce MAH grafts of polyethylenes containing 2.5 wt. % MAH and, in some cases, up to 4 wt. % MAH, graft levels obtained with fumaric acid typically only range from about 0.25 up to about 2 wt. % even with optimal reaction conditions. While useful adhesive blends can be obtained utilizing graft components having reduced levels of functionality, it usually requires the use of a significantly higher concentration of the graft component in the adhesive blends. Since the graft component is generally one of the most expensive components of the adhesive blends, this increases the cost to the processor.
Even though it is possible to produce graft components having higher functionality using MAH, fumaric acid grafts can be used at higher temperatures in food packaging structures. Whereas both MAH and fumaric acid graft copolymers are approved for direct food contact usage under 21 CFR 177.1520, fumaric acid-grafted polyethylene (obtained by the polymerization of ethylene followed by reaction with fumaric acid in the absence of free radical initiators) is sanctioned for use at temperatures exceeding 49° C. while ethylene-maleic anhydride copolymers are not.
It would be highly advantageous if fumaric acid grafts having increased functionality blends were available. It would be even more desirable if functionality levels comparable to those achieved with MAH grafted polyolefins could be obtained with fumaric acid. It would be most advantageous if a process were available whereby both of the foregoing objectives could be realized without directly grafting fumaric acid but rather by converting anhydride functionalized polymers to their fumaric acid counterparts. These and other advantages are realized by the process of the present invention which is described in detail to follow.